1. Field of the Invention
The invention relates in general to optical design, and in particular to systems for removing or reducing distortion to increase efficiency and remove or reduce crosstalk in optoelectronic interconnections.
2. Related Art
Free-space optoelectronic interconnections are known for moving information, in the form of light energy, from one integrated circuit (IC) to another IC, with part of the transmission path being in free space. The conversion of the information from electrical signals to light energy is performed by light sources and the conversion of the light energy back into electrical signals is performed by light detectors.
Free-space optoelectronic interconnections are projected to provide bandwidth densities on the order of a 10.sup.12 bits/sec/cm.sup.2. This bandwidth density exceeds the capabilities of conventional electronics, making free-space optical interconnections an important part of high bandwidth data communications. At this high bandwidth density, if the data rate for each optical source is on the order of 10.sup.9 bits/sec, 1000 light sources per square centimeter may be required. This equates to a grid of light sources with a source-to-source spacing of on the order of 300 microns.
Free-space optoelectronic interconnections ordinarily require that the light from the sources be confined on light detectors. This is ordinarily accomplished by use of an imaging system. As imaging systems are not perfect, the field of optical design concentrates on minimizing the errors introduced by an imaging system. An optical designer minimizes the five (5) third order aberrations of an optical design to meet the required specifications. These third order optical aberrations are: spherical, coma, astigmatism, field curvature, and distortion. While the first four optical aberrations quantify the confinement and shape of the image of an ideal object point in the image, distortion quantifies the location of the image. Distortion is the misplacement of the chief ray of an object point in the image plane, where the chief ray is the ray which travels through the center of the aperture of the optical system. Distortion is usually given as the image location error as a percentage of its position in the image. For example, an imaging system with only one percent distortion would misplace an image of an ideal point located 1 centimeter off-axis by 1% of 1 centimeter or 100 microns. As set forth above, the light sources will be spaced only .about.300 microns apart, as will the light detectors which capture the light. Distortion, which introduces a misplacement of the elements by as much as 1/3 of the grid spacing, is unacceptable for many applications. As described, distortion has a significant detrimental impact on free-space optoelectronic interconnections and should be minimized.
There are three conventional approaches to eliminating the effects of distortion in optical systems: 1) using only on-axis light sources, 2) designing an optical system which is completely symmetric about its aperture, and 3) designing an optical system with sufficiently small distortion to be usable for the application.
FIG. 1 shows a free-space optoelectronic interconnection between integrated circuits 130, 160, which uses only on-axis light sources 110. The integrated circuit 130 includes a plurality of light sources 110 and the integrated circuit 160 includes a plurality of detectors 120. For clarity, only a single light path 150 is shown. Since the detrimental effects of distortion are percentages of the distance the object point, or in this case light source, is from the center of the optical element 140, the resultant misplacement can be made zero if the off-axis distance is zero, e.g., one percent of a zero distance is zero. Thus, placing the light sources on the optical axis of the imaging system results in no apparent distortion in the system. However, if the design is to be applied to a grid of light sources spaced at a small distance, such as 300 microns as described above, then the size of the optical elements can be at most this spacing (e.g., 300 microns). As described in Haney & Christensen, "Performance Scaling Comparison for Free-Space Optical and Electrical Interconnection Approaches," Applied Optics, Vol. 37 No. 14 (May 10, 1998), the small size of the optical elements leads to diffraction limitations which severely limit the distance between the light source and the light detectors. As described in the above reference, this limited "throw distance" mandates the use of repeaters in a large interconnection pattern. The use of repeaters obviates many of the benefits of using free-space optoelectronic interconnections in the first place. This is manifested in an increased power requirement, increased system size (footprint area and volume), and increased interconnection latency.
Another approach is to design an optical system that is completely symmetric about its aperture. FIG. 2 is a diagram of the prior art showing an optical system 260, 270 that is symmetric about its aperture 250. It should be noted that the image plane 220 and object plane 210 do not contain light sources and light detectors, respectively. This symmetric system was designed to image a passive object, with light scattering and/or emanating from it in all directions, onto the image plane. This is an important requirement of the symmetric system, as the aperture is placed at the midpoint between the transmitting lens 260 and the receiving lens 270. This aperture placement dictates that, in general, the light path has an angle that is other than ninety degrees with respect to the object plane 210 and the image plane 220. While this system does not suffer from distortion, it only allows beams at select angles to pass through the aperture. These angles are equal to ninety degrees with respect to the image/object plane only if the distance of the object plane 210 to the transmitting lens 260 equals one half the distance between the transmitting lens 260 and the receiving lens 270 equals the distance from the receiving lens 270 to the image plane 220 equals the focal length of the lens. This configuration is shown in FIG. 3, and is named a 4F imaging system, due to the fact that its overall length is four (4) times the focal length of the identical transmitting and receiving lenses 370, 380.
Active light sources for free-space optoelectronic interconnections are ordinarily designed for light emission orthogonal to the IC in which they are embodied. These ICs are typically placed in the object plane of the lens system; therefore, the light emitted from the light source is typically at a ninety degree angle with respect to the object plane 310. While this is appropriate for a 4F system, as depicted in FIG. 3, it is untenable when the distance between the transmitting lens 370 and the receiving lens 380 is not twice (2x) the focal length. FIG. 4 shows the implications of a light source at a ninety degree angle with the object plane, when the transmitting lens 490 to receiving lens 495 distance is greater that twice (2x) the focal length. It should be noted, firstly, that the light paths cross the optical axis 450 at a position not at the midpoint of the transmitting and receiving lenses (490 and 495 respectively). This optical axis crossing defines the system aperture, so the system is no longer symmetric about its aperture and distortion will be present. Secondly, if a physical constraining aperture were placed at the midpoint of the transmitting and receiving lens, the light paths would necessarily be vignetted. In other words, if the system is made symmetric by placing a limiting physical aperture at the midpoint of the transmitting lens 490 and the receiving lens 495, the limited angular extent of the light source 430 will cause it to miss the aperture entirely and not reach the light detector 440. While this symmetric approach for eliminating distortion in traditional passive optical systems has endured great success, it relies on all angles of light being present from the points in the object plane. The narrow angular extent of active light sources in free-space optoelectronic interconnections does not meet this requirement and therefore requires a new approach for eliminating distortion.
The third approach to eliminating the effects of distortion is to minimize, not eliminate, the distortion over the image plane through the addition of lens surfaces to the imaging optics. To achieve reliable interconnects for dense optoelectronic interconnections across a 1-centimeter plane the distortion should be at a fraction of one percent. This requirement is stringent, and typically the addition of several optical surfaces is required. Additional optical surfaces, in turn, increase the complexity, weight, and cost of the optical system.